Rutile boule and method of making the same



2,715,070 RUTILE BOULE AND ISVIETHOD OF MAKING THE AME Charles H. Moore, Jr., Indianapolis, Ind., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey it a No Drawing. Application April 29, 1954, Serial No. 426,583

7 Claims. (Cl. 10642) The present invention relates to rutile single crystal boules modified with respect to colorand certain other physical properties, and to methods for the production of such boules.

Rutile is one of the three known crystal modifications of titanium dioxide. When substantially pure, a massive single crystal of rutile has gem-like properties with a very light straw color and reflectance, refraction and brilliance greater than that of a diamond. The rutile single crystal is, however, not as hard as a diamond. By means of the present invention rutile single crystals nearly water-white color and increased hardness andtoughness, i. e. are less brittle or non-brittle. In addition, the present invention permits finishing the boule by reoxidation in considerably less time than is required for a pure rutile single crystal boule.

The principal object of the present invention is the preparation of rutile single or stal boules having nearly Water-white color and a hardness greater than that of ration of a finished oxidized pure rutile single crystal boule. These and other objects of the present invention will become apparent from the following description.

In its broadest aspectthe present invention contemboule containing a small a hardness and toughness greater than that of a pure rutile single crystal, prepared according to the method of the present invention by progressively fusing a mixture of finely-divided TiO2 containing a small amount of alumina in a flame carrying an excess of an oxidizing gas and thereafter oxidizing the boule so formed.

In this description of the present invention the word boule is used in its currently accepted meaning to denote a characteristic shape or form of an artificially prepared massive single crystal having a rounded end, or meniscus, a more or less rod-like body portion and a tapering end, giving an over-all appearance in profile of an almond, or carrot.

This application is a continuation-in-part of my copending application Serial No. 89,816 filed April 26, 1949, now abandoned.

The general method to which the present invention relates is that described in co-pending application Serial No. 54,562, filed October 14, 1948, now abandoned, for the preparation of pure rutile single crystal boules.

and possesses an open structure with units capable of being rapidly melted. A TiOz having an ultimate unit particle size of approximately 0.1 micron has proved especially satisfactory. In general, material having an 2,715,070 Patented Aug. 9, 1955 2 average particle size abovevabout 5 microns should be avoided because such under the conditions of the invention. method for preparing a TlOZ starting material especially adapted for the production of rutile single crystal boules is described in co-pending application, Serial No. 54,578 filed October 14, 1948, issued as Patent No. 2,521,392, September 5, 1950, wherein a doublesalt, ammoniumtitanium-sulphate is calcined so that all of the ammonia and sulfate are removed by volatilization.

According to the present invention the small amount of alumina may be mixed with monium-titanium-sulphate prior to calcination.

The amount of alumina, A1203, to be added to the TiOz starting material is very small and should not exceed about 0.1%, calculated asAl2O3 based on the weight of the TiO2. A1203, said A120 present in amount from about 0.005% to about 0.1%. The preferred amount lies between 0.05%. When the amount of alucolor are not substantially affected.

In progressively fusing the, starting mixture of TiOz and A12 the amount required to combine with the hydrogen is introduced through the outer tube. Preferably, the ratio of oxygen to hydrogen by volume should be about 1:1 and should not be less than about 8.5 parts of oxygen to 9.5 parts hydrogen by volume.

The flame temperature should be carefully controlled during the formation of the boule. It has been found that satisfactory results are obtained when the temperature is held between about 1825 C. and 1900 C. around the area where the boule is formed. Rutile melts at about 1820 C. and therefore it is necessary to operate so that the temperature at the top of the boule is somewhat above the melting point, e. g. 1825 C. If the temperature is in excess of 1900 C. an excessively large portion of the boule melts at the top and the molten material boils and runs over the sides of the boule.

particles do not satisfactorily fuse;

A preferred the TiO2 starting material.

The boule will consist of TiO2 and such conditions the formation of lower, or sub-oxides of titanium, is so great that mixtures of small crystals of various oxides of titanium are produced rather than a single crystal boule.

Even when operating with an excess of oxygen the boule as produced has a generally deep blue-black color indicative of some deficiency in oxygen. Therefore, in order to produce the nearly water-white boule of the present invention, the blue-black boule has to be reoxidized.

The rcoxidation of the boule is preferably carried out by heating the blue-black boule in an oxidizing atmosphere, for instance, air or oxygen. The temperature of the treatment should be within the range from about 650 C. to about 1500 C. It has been found that at temperatures to any extent below 650 C. oxygen will not be appreciably incorporated into the rutile single crystal. There is a decided loss in brilliance, lustre and fire when the treatment is carried out above 1500" C. and at that temperature the rate of oxygen incorporated into the rutile crystal is excessively rapid and ditficult to control. Preferably the oxidation should be carried out at about 1100 C. to 1300 C. to properly anneal the boule. The heating should be continued until the boule obtains the maximum degree of lightness of color. This time will usually be about one quarter of that required to fully oxidize a pure rutile boule.

As produced, the alumina containing rutile boule has a more or less frosted outer surface. When split, the interior surfaces of the pieces are vitreous and shiny and nearly water-white. They may be cut and polished to form beautiful gems, more brilliant than diamonds and having a white color compared to the characteristic straw or light yellow color of pure rutile.

A rutile boule made according to this invention containing 0.01% alumina was tested for hardness using a Tukon tester as described in the Handbook of the American Society of Metals, 1948 edition, page 96. A Knoop indenter was employed carrying a load of 400 grams. The Knoop number indicating hardness, obtained on a boule of this invention compared to that obtained on a pure rutile boule is shown in the table below.

TABLE Knoop hardness [400 g. load applied parallel to axis] occurred at 400 g. load).

r titania and between 0.005%

As shown above the alumina containing boule gave a Knoop-number of 1030 with a 400 g. load. The pure rutile boule splintered and cracked under a 400 gram load showing brittleness and lack of toughness compared to the alumina containing boule. The pure rutile boule tested under a 200 gram load showed a hardness of 900 calculated on the same basis as the 400 gram load used in testing the alumina containing boule. Therefore it is evident that the alumina containing boule is tough, non-brittle and shows an increased hardness compared to a pure rutile boule.

The foregoing description has been given for purposes of illustration, but the invention is not to be limited thereto and various modifications and variations within the scope of the appended claims may be employed.

I claim:

1. A rutile single crystal boule consisting of titanium dioxide and alumina, said alumina present in amount from 0.005% to 0.1% by weight.

2. A rutile single crystal boule consisting of titanium dioxide and alumina, said alumina present in amount from 0.005% to 0.05% by weight.

'3. A method for producing a rutile single crystal boule which comprises forming an intimate mixture consisting of finely-divided titanium dioxide and from 0.005 to 0.1% alumina by weight, and fusing said mixture in an oxidizing atmosphere at a temperature of from 1825 C. to 1900 C. to form a single crystal boule.

4. A method for producing a substantially White single crystal boule which comprises forming an intimate mixture consisting of finely-divided titanium dioxide and from 0.005% to 0.1% alumina by weight, and fusing said mixture in an oxidizing atmosphere at a temperature of from 1825 C. to 1900 C. to form a single crystal boule and heating said boule in an oxidizing atmosphere at a temperature of 650 C. to 1500 C. until a substantially white color is obtained.

5. A substantially white rutile single crystal consisting of titaniumdioxide and alumina, said alumina present in amount from 0.005% to 0.1% by weight.

6. A single crystal of synthetic rutile consisting of titania and between 0.03% and 0.1% of alumina.

7. A single crystal of synthetic rutile consisting of and 0.1% of alumina.

No references cited. 

1. A RUTILE SINGLE CRYSTAL BOULE CONSISTING OF TITANIUM DIOXIDE AND ALUMINA, SAID ALUMINA PRESENT IN AMOUNT FROM 0.0005% TO 0.1% BY WEIGHT 